RAD Data comm MPW-1 Specifications download pdf (Page 27)

Installation and Operation Manual Chapter 1 Introduction

MPW-1 MP-4100 Ver. 2.0 Functional Description 1-15

• The jitter buffer is filled by the incoming packets and emptied out to fill the

TDM stream. If the PSN jitter exceeds the configured jitter buffer size,

underflow/overflow conditions occur, resulting in errors at the TDM side:

 A jitter buffer overrun occurs when it receives a burst of packets that

exceeds the configured jitter buffer size + packetization delay. When an

overrun is detected, MPW-1 clears the jitter buffer, causing an underrun.

 A jitter buffer underrun occurs when no packets are received for more

than the configured jitter buffer size, or immediately after an overrun.

When the first packet is received, or immediately after an underrun, the buffer is

automatically filled with a conditioning pattern up to the PDVT level in order to

compensate for the underrun. Then, MPW-1 starts processing the packets and

empty out the jitter buffer toward the TDM side.

To minimize the possibility of buffer overflow/underflow events, two conditions

must be fulfilled:

• The buffer must have sufficient capacity. For this purpose, the buffer size can

be selected by the user in accordance with the expected jitter characteristics,

separately for each pseudowire, in the range of 0 to 200 msec.

• The read-out rate must be equal to the average rate at which frames are

received from the network. For this purpose, the read-out rate must be

continuously adapted to the packet rate, a function performed by the

adaptive clock recovery mechanism of each packet processor.

Adaptive Timing

The receive path of each pseudowire must use a clock recovery mechanism to

recover a clock signal at the original payload transmit rate used at the far end.

This mechanism is referred to as

adaptive clock recovery mechanism

The adaptive clock recovery mechanism estimates the average rate of the

payload data received in the frames arriving from the packet-switched network.

Assuming that the packet-switched network does not lose data, the average rate

at which payload arrives will be equal to the rate at which payload is transmitted

by the source.

Generally, lost packets, as well as packets that did not arrive in the correct order,

are replaced by special dummy packets. However, for CESoPSN and SAToPSN,

packets can be reordered.

The method used to recover the payload clock of a pseudowire is based on

monitoring the fill level of the selected pseudowire jitter buffer: the clock

recovery mechanism monitors the buffer fill level, and generates a read-out clock

signal with adjustable frequency. The frequency of this clock signal is adjusted so

as to read frames out of the buffer at a rate that keeps the jitter buffer as near

as possible to the half-full mark. This condition can be maintained only when the

rate at which frames are loaded into the buffer is equal to the rate at which

frames are removed. Therefore, the adaptive clock recovery mechanism actually

recovers the original payload transmit clock.

The performance of the clock recovery mechanism can be optimized for the

operating environment, by specifying the following parameters:

Not

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RAD Data comm MPW-1 Specifications Page 27

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